Lubricating oils include, for example, industrial gear oils, engine oils, lubricating oils for refrigerators, lubricating oils for textile industry and lubricating oils for rolling mills.
Recently, the industrial gear oils have been desired to keep the lubricating properties and the detergency at higher temperature regions, as the environmental conditions under which various industrial machines are used have come to be more severe. Especially in baking paint process or baking food process, those oils have been desired to have higher performances in the lubricating properties and the detergency. In those areas, lubricating oils of synthetic hydrocarbon type, carboxylic ester type or glycol type have been conventionally employed.
The synthetic hydrocarbon type oils and carboxylic ester type oils, however, have such problems that they are insufficient in the lubricating properties and they cannot function as the lubricating oils at high temperatures because they are carbonized when heated for a long period of time. On the other hand, the glycol type lubricating oils have such a merit that they are hardly carbonized even when heated for a long period of time, but they are , insufficient in the lubricating properties and have high moisture absorption properties (hygroscopicity), so that these oils are desired to be improved in the lubricating properties and the resistance to moisture absorption.
The engine oils have been required to have lubricating properties and detergent-dispersing properties at higher temperatures for a long period of time, in accordance with enhancement in the performance of the automobile engines. In the case of using additives to comply with those requirements, the additives are necessarily used in a large amount, and hence a precipitation of a curdy (mayonnaise-like) sludge takes place. Further, a co-use of the synthetic hydrocarbon type oil or carboxylic ester type oil and a mineral oil as a base oil has been conventionally tried. However, the engine oil thus obtained is insufficient in both the lubricating properties and the detergent-dispersing properties at high temperatures for a long period of time.
Differently from the above-mentioned lubricating oils for automobile engines, namely, those for four-cycle engines, the lubricating oils for two-cycle engines are added to gasoline and subjected to combustion in the two-cycle engines, so that the detergency is particularly important for the lubricating oils for two-cycle engines. As the lubricating oils for two-cycle engines, there have been heretofore used a caster oil, polybutene, etc., but they are not sufficient both in the lubricating properties and the detergency.
The automobile gear oils, particularly gear oils for ATF, are necessary to be decreased in the friction coefficient and moreover to be reduced in a change of the friction coefficient with time. Therefore, an anti-friction agent or a friction-adjusting agent has been conventionally added to decrease the friction coefficient. However, the automobile gear oils containing these additives involve such a problem that a friction coefficient becomes larger during use.
As the lubricating oils for textile industry, those of carboxylic ester type or glycol type have been heretofore used, but they are not satisfactory both in the lubricating properties and the detergency.
As the lubricating oils for rolling mills, those containing beef tallow as the host component have been conventionally used. Such lubricating oils show good lubricating properties and excellent rolling efficiency. However, the detergency of these oils is markedly bad, so that a step of washing off the residual beef tallow is essential. Also used as the lubricating oils for rolling mills are those of carboxylic ester type, but these oils show poor lubricating properties, resulting in poor practicability, although they are very excellent in the detergency.
With the alteration of a refrigerant gas for refrigerators to R-134a (CH.sub.2 F--CF.sub.3) which is nondestructive to the ozone layer, mineral oils or alkylbenzene compounds having been heretofore used as the lubricating oils for refrigerators have become unusable, because they have no compatibility with the refrigerant gas. Hence, glycol ether type lubricating oils have been now developed as the lubricating oils for refrigerators using the above-mentioned refrigerant gas.
For example, U.S. Pat. No. 4,755,316 discloses a composition for a compression refrigerator which comprises tetrafluoroethane and polyoxyalkylene glycol having a molecular weight of 300 to 2,000 and a kinematic viscosity at 37.degree. C. of about 25 to 150 cSt.
However, there are pointed out such defects that this glycol ether type lubricating oil is generally insufficient in heat stability and high in hygroscopicity, and moreover it shrinks a rubber sealing material such as NBR to increase the hardness.
In refrigerators for automobile air conditioners a through-vane type rotary compressor which can make a size of the compressor smaller and increase power thereof has been used in recent years. As the lubricating oils for the through-vane type rotary compressor, those having a high viscosity is more desired than those having sealing properties and friction resistance. However, when compounds having glycol ether structure are increased in the molecular weight to have a high viscosity, the compatibility thereof with the ozone layer-nondestructive R-134a is generally deteriorated, so that such compounds cannot be employed from the structural viewpoint.
Further, carboxylic ester type lubricating oils called "polyol ester" and "hindered ester" have been developed recently as the lubricating oils for refrigerators where the ozone layer-nondestructive hydrogenated fluorocarbon (HFC) is used as a refrigerant. However, these lubricating oils are hydrolyzed or heat-decomposed to produce a carboxylic acid, and thus produced carboxylic acid causes a phenomenon of corrosion and abrasion of metals or a phenomenon of copper plating in the refrigerator. Therefore, an endurance of the refrigerator comes to be a problem in the case of using the lubricating oils stated above. Moreover, a part of the carboxylic acid produced by the hydrolysis or the heat decomposition is further decomposed under severe use conditions to generate a carbon dioxide gas. This carbon dioxide gas has non-condensation properties in an ordinary refrigerator system where fluorocarbon, chlorofluorocarbon or hydrogenation product thereof is used as a refrigerant, and hence decrease of refrigeration efficiency and temperature rise in the compression step are induced.
The ozone layer-nondestructive hydrogenated fluorocarbon (HFC) also includes concretely R-152a as well as the aforesaid R-134a. Also employable as the refrigerant is hydrogenated chlorofluorocarbon (HCFC) having a small destructive force to ozone, and this hydrogenated chlorofluorocarbon includes concretely, for example, R-22, R-123 and R-124. These hydrogenated chlorofluorocarbons are used singly or in combination with the hydrogenated fluorocarbons (HFC).
The present inventors have earnestly studied for the purpose of obtaining lubricating oils which are excellent in lubricating properties, detergency, electrical insulation properties and compatibility with both the hydrogenated fluorocarbons (HFC) and the hydrogenated chlorofluorocarbons (HCFC), and further which can prevent generation of the carboxylic acid and carbon dioxide gas. As a result, the present inventors have found that lubricating oil compositions excellent in the above-mentioned various properties can be obtained by blending a specific polycarbonate and at least one compound selected from the group consisting of an epoxy compound, a phenol compound, a sulfur compound and an amine compound in the specific amounts, or by blending a specific polycarbonate derived from sugars and a phosphorous triester compound in the specific amounts, and they have accomplished the present invention.